Polymorphisms in the B lymphoid tyrosine kinase (BLK) gene have been associated with autoimmune diseases, including systemic lupus erythematosus, with risk correlating with reduced expression of BLK. How reduced expression of BLK causes autoimmunity is unknown. Using Blk(+/+) , Blk(+/-) , and Blk(-/-) mice, we show that aged female Blk(+/-) and Blk(-/-) mice produced higher anti-dsDNA IgG Abs and developed immune complex-mediated glomerulonephritis, compared with Blk(+/+) mice. Starting at young age, Blk(+/-) and Blk(-/-) mice accumulated increased numbers of splenic B1a cells, which differentiated into class-switched CD138(+) IgG-secreting B1a cells. Increased infiltration of B1a-like cells into the kidneys was also observed in aged Blk(+/-) and Blk(-/-) mice. In humans, we found that healthy individuals had BLK genotype-dependent levels of anti-dsDNA IgG Abs as well as increased numbers of a B1-like cell population, CD19(+)CD3(-)CD20(+)CD43(+)CD27(+), in peripheral blood. Furthermore, we describe the presence of B1-like cells in the tubulointerstitial space of human lupus kidney biopsies. Taken together, our study reveals a previously unappreciated role of reduced BLK expression on extraperitoneal accumulation of B1a cells in mice, as well as the presence of IgG autoantibodies and B1-like cells in humans.